Performance modeling of a hybrid Diesel generator-Battery hybrid system Central University of Technology

Energy Postgraduate Conference 2013

Presentation outline

• Introduction

• System Layout

• Design variables

• Optimal operation control

• Operation strategies

• Conclusion

IntroductionSmall rural area electrification challenges:

Reliable electrical power supply lacking, high cost of AC grid extension, rough

topography.

Diesel generator in rural electrification:

• Advantages: Low initial capital costs and generate electricity on demand.

• Disadvantages: Costs (operation fuel, maintenance running, replacement

lifespan), Environmental impacts (noise and pollutions).

Hybrid Diesel – Battery system:

By optimally designing the hybrid generator-battery system and applying optimal

operation control strategies, the overall running costs and well as the

environmental impacts can be sensibly reduced.

This paper will propose an approach for optimal operation control of a

hybrid Diesel generator - battery system with the aim of minimizing the operation

cost while meeting the load energy requirements.

The paper will be limited at the problem formulation and the development of

the mathematical model for the performance of all the hybrid system’s

components.

System Layout The proposed hybrid system is

composed of a diesel generator, a

battery storage system, an

inverter, a charge controller as well

as AC and DC loads.

The hybrid system model is

based on a description of current

flow from the different sources,

taking into account the losses and

the impact of the operating

decisions along the way up to the

loads.

Design variablesDiesel generator:

The instantaneous output current from the DG depends on variables such as the type or the

size of the DG used (XDG, Size, Type). It can be expressed as:

%max, DGDGDG XII

Battery storage system

The instantaneous output current from the whole battery bank depends on variables such as

the type or the size of the battery used (XBat, Size, Type), the number of battery strings in parallel

XBat. It can be expressed as: )%()(max,,)( tBattBattBat XII Inverter

The size of the inverter XInv is expressed in terms of its AC output power.

InvInvInInvOutInv XPP

If XR, DC is the percentage of current from the DC bus (IDC) redirected to the DC load, the

input current to the inverter can be expresses as (1-XR, DC). XR, DC is an operation variable.

BusAC

BusDCInvDCRBusDCOutInv V

VXII

,

,,, )1(

Transfer switch

Its position (XS) is an operation decision variable. If XS=0, all the AC load is supplied by the

DG and the inverter is off. If XS=1, all the AC load is supplied by the DC bus through the

inverter. Battery charger

The size of the battery charger XBC in terms of its DC output power. BCBCInBCOutBC XPP

The percentage (XR, AC) of current from the DG (IDG) redirected to the battery charger is:

BusACBC

BusDCOutBCACRDG V

VIXI

,

,,

Optimal operation controlObjective function:

The aim is to minimize the fuel consumption cost from the DG during the operation time. This can be expressed as:

Where: N is the number of sampling intervals within the operation range or period of the system. a ,b, c are coefficients. p is the price of the fuel. j is the jth sampling interval. IDG,j is the output current from the DG at jth sampling interval.

N

jjDGjDG pcbIaI

1,

2, )(min

Constraints:

• Load balance

Unmet

Balance

Excess

II SupplyiLoadi

:0

:0

:0

,,

• Variable limits

Operation limits]1,0[

1;;0 ,,%

S

DCRACRDG

X

XXX

Design limits max,,,,,

min,, TypeSizeijTypeSizeiTypeSizei XXX

• Battery limits max,

minBjBB SOCSOCSOC

Operation strategies

• Diesel generator favourite: If the DG (CF) is between some preset values

[CFX1%, CFX2%] and the battery state of charge (SOC) is between some preset

values [SOCX1%, SOCX2%], then the DG is preferred.

• Battery system favourite: If the battery state of charge (SOC) is between

some preset values [SOCY1%, SOCY2%] and the DG capacity factor (CF) is between

some preset values [CFY1%, CFY2%], then the battery is preferred.

Conclusion The purpose of this paper was to propose an approach for optimal operation

control of a hybrid Diesel generator - Battery system with the aim of meeting the

load energy requirement while minimizing the operation costs all through the life

of the system. The sizing optimization variables, operation decisions variables to

be optimized at time t, as well as the constraints for the optimal sizing and

operation control of the hybrid system have been described. The time

independent operation strategies can be chosen by making a search through

potential settings for the hybrid system optimal operation control.

For further studies, the developed model can be integrated to the hybrid

system’s total life cycling cost and then be used by any of the traditional

(gradient-based methods) or modern (artificial intelligence) optimization

algorithms to find the best solution for the system.